Resistance against erosion wear is normally related to the hardness of the wear part. Some articles are subject to solid particle erosion in which particles of various sizes and hardness are propelled at various angles against the surface of the articles. For example, a car traveling in the desert during a wind storm will encounter various size solid particles of sand traveling at various velocities hitting the car. If the size of the particles is large and the velocity of the particles is high, the coating on the car could be chipped or pitted. In turbomachines which operate in a dust environment, this solid particle erosion is a severe problem. Recently, physical and chemical vapor deposited coatings, such as titanium nitride coatings and zirconium nitride coatings, have been used to provide a protective layer having good hardness characteristics. These coatings have been found to have good erosion resistance to Al.sub.2 O.sub.3 and SiO.sub.2 particles at both high and low impact angles. Although these coatings have high hardness characteristics, they exhibit inherently brittle behavior and their erosion resistance at normal impact decreases markedly with increasing hardness and particle size of the erodent. It has been observed that dynamic impact of solid particle erodents onto a coated surface of an article can form lateral and/or median cracks around the impact site. Median cracks are responsible for the strength degradation of the material while lateral cracks, which grow from the center of impact parallel to the substrate surface and then propagate through the coating surface, account for most of the material loss during solid particle impact erosion. The solid particle impact erosion of these coatings at a 90.degree. impact angle is due primarily to brittle fracture. Thin coatings are more suspectible to spalling and exposure of the substrate which may lead to premature failure of the article. When coatings applied by conventional techniques are exposed to particle impact, pinholes and/or lateral spalling pits generally result in the coating. Once the coating material is cracked, additional impact by even relatively small particles will cause furrowing or grooves in the coating material. In a turbomachine, this furrowing can greatly effect the overall performance of the turbomachine.
Based on the elastic-plastic theory, toughness and hardness are the dominant properties controlling the erosion behavior. Higher hardness is believed to increase erosion resistance at both low and high impingement angles while higher toughness reduces the vulnerability to brittle fracture and markedly increases 90.degree. erosion resistance. An erosion resistant coating needs to be simultaneously hard and tough. However, hardness and toughness are generally in opposition in hard materials. Higher hardness is usually associated with greater brittleness. Multilayer hard compound materials have been found to have simultaneously high hardness and high toughness. The high hardness is an inherent property of hard compounds and the high toughness is attributed to the formation of a coherent or partly coherent interface boundary between two different hard compound layers. For example, it has been found that a TiC/TiB.sub.2 multilayer coating has a better wear resistance than either the TiC or TiB.sub.2 single layer. In cutting tool applications, the multilayer coated tool bits, having a TiC/A1.sub.2 O.sub.3 /TiN three layer coating, or a two layer coating in which one layer is a nitride, carbide, boride, or silicide of a metal of Group VI and the other layer is a nitride or carbide of a metal of Group VI, have shown good performance in wear resistance.
An object of the present invention is to provide a multilayer coated substrate having good erosion and/or wear resistance characteristics to solid particle impact.
Another object of the present invention is to provide a multilayer coated substrate in which each of the layers comprises a nitride-containing compound and in which the nitrogen content of one layer is different than the nitrogen content of an adjacent layer.
Another object of the present invention is to provide a multilayer coated substrate in which each of the layers is a titanium nitride containing compound and in which the nitrogen content of one layer is less than or more than the nitrogen content in an adjacent layer.
Another object of the present invention is to provide a multilayer coated substrate comprising at least three layers in which each layer comprises a titanium nitride-containing compound having an atomic percent of nitrogen from 33% to 55% and wherein the nitrogen content of one layer has at least 2 atomic percent of nitrogen more than the nitrogen contained in an adjacent layer.
Another object of the present invention is to provide a multilayer coated substrate in which one layer is Ti.sub.2 N mixed with titanium nitride having an atomic percent of nitrogen of about 40% and an adjacent layer is titanium nitride having an atomic percent of nitrogen of 40% to 55% and wherein the nitrogen content of one layer has at least 2 atomic percent of nitrogen more than the nitrogen contained in the adjacent layer.
The above and further objects and advantages of this invention will become apparent upon consideration of the following description.